The present invention relates to a second harmonic generator which utilizes Cherenkov radiation that is useful as a source of coherent short-wave light. Such light is useful for optical reading or recording, such as in optical disks, laser printers or color printers. Such devices benefit from light having a higher frequency, and correspondingly, a lower wavelength. Shorter wavelength light provides for example, increased resolution in laser printers, and increased data density in optical disks.
It has been recognized that notwithstanding the limited optical frequency spectra obtainable from current semiconductor laser devices, such limitations may be remediable by utilization of a phenomenon in which waves having a phase velocity greater than the phase velocity of a medium radiate energy into the medium at a given angle (Cherenkov angle).
There has heretofore been known technology for generating second harmonic of Cherenkov radiation by forming a non-linear optical organic material into fiber form and inputting a coherent primary light thereto, as reported in the Nonlinear Optical Materials--Extended Abstract, 1985, p. 97. The radiant beam, however, has the shape of a cone and cannot be focused into a good spot, presenting a problem in utilization of the radiant beam, especially in light of the fact that only a small fraction of total laser energy forms the radiant beam.
Generation of second harmonics by Cherenkov radiation from waveguides using lithium niobate (LiNbO.sub.3) as a substrate, has been reported in CLEO '87 Technical Digest, pp. 198. This method has many advantages in that the secondary light having a wavelength one-half that of the fundamental light can be generated. This method simultaneously maintains a high conversion efficiency. Phases of the primary and secondary light also have different wavelengths which can be matched relatively easily. With this method, however, a radiation mode is established from a narrow line-shaped waveguide into the substrate, whereby the light diverges in the direction of radiation while the light is collimated in parallel in the direction at right angles thereto. Accordingly, the wave front is distorted. Therefore, the light cannot be focused to a point maintaining diffraction limited wave front accuracy, which is a fatal defect that hinders the application to optical recording or reading, such as optical disks or laser printers.
Such an earlier method is shown in FIG. 4. Therein, the substrate of the waveguide consisted simply of a block having a flat surface. The beam that has passing through the substrate develops large aberration, but no means for correcting the aberration had ever been employed.
The subject invention remedies all the above-referred problems, and others, and recognizes in that the above-mentioned technologies, a problem resided in that attention had not been sufficiently given to beam formation.